The present invention relates to a method and a treating solution for phosphating metal surfaces (hereinafter, the term "a phosphating method and solution" is used for this method and solution, respectively.) and, in detail, when a product consisting of steel and/or zinc-plated steel combined with an aluminum alloy is coated with cationic electrocoating, with an attempt for elevating the coating finish and rust-preventive property, the present invention relates to a phosphating method for forming on a metal surface a phosphate film, of which fundamental component is zinc phosphate, and also to a phosphating solution which is used for said phosphating method.
It is so far widely known that, by phosphating a metal surface, a film which is appropriate as an under coating for cationic coating is made.
Although steel and zinc-plated steel are common as the metal material, in recent years a product consisting of combination of steel and/or zinc-plated steel with an aluminum alloy are becoming available. For example, although the automobile bodies all have been so far made of steel plates, an attempt to replace a part of the bodies with an aluminum product has been carried out. Also, in replacement of the steel plates, a zinc-plated steel plate is in an increasing use and also, use of a zinc-plated steel plate combined with an aluminum alloy is increasing. Thus, has been desired a phosphating method capable of simultaneous treating of a metal surface consisting of the steel and/or zinc-plated steel combined with an aluminum alloy.
There have been proposed various conditions for a method and solution for phosphating the steel and zinc-plated steel, under which the coating finish, adhesion, and rust-preventive property etc. are elevated, and for example, which are opened in Japanese Official Patent Provisional Publications, showa 57-152472 and 59-35681.
However, in these kinds of phosphating methods for steel and zinc-plated steel, when the steel and/or zinc-plated steel are simultaneously treated with an aluminum alloy, an aluminum ion elude the aluminum alloy and accumulate in a treating bath, and the phosphate film on the surfaces of steel and zinc-plated steel is not normally made owing to the accumulated aluminum ion. Also, an uniform film is not made on the surface of an aluminum alloy.
In order to solve the above-mentioned problems, there has been proposed a method in which, to prevent the elusion of an aluminum ion into the phosphating solution, converting treatment of the aluminum alloy into a passive state such as the chromating is beforehand carried out in a separate process and the steel or zinc-plated steel is simultaneously with an aluminum alloy treated with phosphating, and which is opened in Japanese Official Patent Provisional Publication, showa 61-96074. However, this method needs a converting process of an aluminum alloy into a passive state in addition of the phosphating process and also, properties which are satisfactory as an under coating for cationic coating have not been displayed on the surfaces of steel and zinc-plated steel as well as on the surface of an aluminum alloy.
Furthermore, there have been proposed phosphating methods in which phosphating solutions containing a fluorinated compound are used, and which are opened in Japanese Official Patent Provisional Publications, showa 63-15789 and 64-68481.